1.. Field of the Invention
The present invention relates to semiconductor devices such as dynamic random-access memories, which have functions that adjust the levels of internal voltage sources.
The present application claims priority on Japanese Patent Application No. 2007-177300, the content of which is incorporated herein by reference.
2.. Description of the Related Art
Recently developed semiconductor memory devices such as dynamic random-access memories (DRAM) may operate based on levels of internal voltage sources thereof, which should be individually adjusted with respect to individual products.
Various types of semiconductor memory devices having functions that adjust the levels of internal voltage sources have been disclosed in various documents, such as Patent Document 1 and Patent Document 2.                Patent Document 1: Japanese Unexamined Patent Application Publication No. H05-56559        Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-85971        
Patent Document 1 teaches a semiconductor device having a plurality of voltage generation circuits for generating different levels of voltages based on an externally supplied voltage, wherein one of multiple voltage generation circuits is selected based on the operational characteristics of logic gates.
Patent Document 2 teaches a semiconductor memory device, which selectively turns on multiple transistors connected in series so as to produce a desired voltage based on on-resistance thereof.
Various methods have been developed to adjust dimensions of drive transistors by way of current controls of voltage generation circuits incorporated in conventionally-known semiconductor memory devices such as DRAM. One method is to change the dimensions of drive transistors. Another method is to additionally provide other drive transistors. In addition, various methods have been developed to finely adjust the dimensions of transistors. That is, one method is to change the dimensions of transistors. Another method is to additionally provide other transistors having very small dimensions. To achieve significant adjustment regarding the dimensions of transistors, it is necessary to additionally provide transistors whose dimensions are greater than those of transistors used in fine adjustment; alternatively, it is necessary to additionally provide numerous transistors (whose dimensions are identical to those of transistors used in fine adjustment), the number of which is far more than the number of transistors used in fine adjustment.
It is necessary for recently developed semiconductor memory devices such as DRAM to have low power consumption and high precision of dimensions because they must operate normally without interruption. In the aforementioned methods for additionally providing transistors having very small dimensions, in particular, in significant adjustment, it is necessary to perform a first step of additionally providing transistors whose dimensions are greater than those of transistors used in fine adjustment or a second step of additionally providing numerous transistors (whose dimensions are identical to those of transistors used in fine adjustment), the number of which is far more than the number of transistors used in fine adjustment. The difference between the first and second steps may degrade the partial precision of adjustment in prescribed processes, thus degrading the entire precision of adjustment.
The main factor in degrading the precision of adjustment is due to very small currents, which may not be affected by dimensions of drive transistors, within currents applied to drive transistors. Conventionally, they are very small and negligible; however, they have been recently highlighted as a negative influence because of a reduction of currents of adjustment circuits. They may be easily varied in manufacturing processes; hence, the conventional technology suffers from a problem in that it is difficult to secure an adequate precision of operation in semiconductor memory devices using adjustment circuits designed based on pre-estimated currents flowing therethrough.